The present invention relates to an adaptive echo canceller which cancels an echo while estimating the transmission characteristic of an echo path through the use of a received signal and an echo signal.
The principle of an echo canceller is to estimate the transmission characteristic of an echo path using a received signal and an echo signal, form a pseudo-echo signal on the basis of the estimation result and subtract the pseudo-echo signal from a true echo signal, thereby cancelling an echo. The echo canceller basically excels conventional echo suppressors but has a defect that since it successively estimates the transmission characteristic of the echo path, a misestimation is likely to occur when the noise level of a signal or the like of a near-end talker is high relative to an echo signal level as in a case of double talking.
To avoid such a defect, there have been proposed a method using a double talking detector and a method which employs two echo path models, successively corrects only one of the models and sets, as a new echo path model, the model with a smaller error signal (Kazuo Ochiai, et al: "Echo Canceller with Two Echo Path Models", IEEE Trans Vol. COM-25, No. 6, June 1977, pp. 589-595). The former is an application to an echo canceller of a device widely employed in echo suppressors, but the detector of this method is basically insufficient in sensitivity and poses a problem that a misestimation is already formed before a double talking is detected. In contrast thereto, the latter exhibits a relatively excellent characteristic even in a case of double talking but has the drawback that it cannot follow up a gentle variation in the SN ratio.